The present invention relates to a wheelspeed sensing system for use in a brake control system and more particularly to a means for digitally measuring the period of a periodic signal having a period related to wheelspeed.
One important class of modern brake control systems monitors the rotation of one or more wheels and modifies the braking force applied to the wheel as necessary for proper braking. Such brake control systems are in widespread use in commercial and military aircraft, and are in growing use in business aircraft and ground transportation vehicles. In use, these systems provide improved braking control and reduced stopping distances, and as such, they are important safety components of many modern vehicles.
One of the central elements of this type of brake control system is the wheelspeed transducer and associated circuitry. An accurate measure of wheelspeed is an important first step in detecting and then controlling braking conditions such as wheel skid. In the past, many brake control systems have relied on analog circuits to generate an analog parameter representative of wheelspeed. For example, one approach used in the past has been to process the periodic output signal of a wheelspeed transducer to generate a series of impulse signals which correspond in frequency with the periodic signal produced by the transducer. The impulse signals are then applied to an electrical charge integrator, such as a capacitor, and the voltage on the capacitor is used as a measure of wheel velocity.
The prior art analog approach exhibits certain limitations and drawbacks. In that the impulse signals are integrated, multiple periods of the transducer signal are often required to accurately indicate changing wheelspeed. Furthermore, this approach utilizes analog circuits which are subject to inaccuracies due to drift under certain circumstances. Moreover, these analog approaches are not well suited for use with brake control systems employing digital processors. The analog wheelspeed value must be digitized before it can be input into a digital processor, and the time delays required for precision analog to digital conversion can be a drawback in some applications.